In optical networks using the WDM (Wavelength Division Multiplexing) technique, mapping of various kinds of signals and transmission of large-capacity signals are required in order to provide various services.
To operate and manage such optical networks, an OTN (Optical Transport Network), for example, is standardized by the ITU-T (International Telecommunication Union Telecommunication Standardization Sector). A frame signal (hereinafter simply referred to as a “frame”) for optical transfer is recommended in G. 709 “Interfaces for the Optical Transport Network.”
FIG. 7 illustrates the format of an OTN frame. An OTN frame 100 includes an overhead area 101, a payload area 102, and a FEC (Forward Error Correction) area 103.
The overhead area 101 includes a FAS (Frame Alignment Signal) field, an OTU (Optical channel Transport Unit)k OH (Overhead) field, an ODU (Optical channel Data Unit)k OH field, and an OPU (Optical channel Payload Unit)k OH field.
The FAS field is a pattern indicative of the head of the OTN frame. The FAS field and OTUk OH field is an overhead field used for frame synchronization and the management of connection and quality for OTU. The ODUk OH field is an overhead field used for the management of connection and quality for ODU.
A client signal for providing various services is mapped into an OPUk payload area which is the payload area 102. Alternatively, a plurality of ODUjs (j<k) the signal rate of which is lower than that of an OPUk are multiplexed and mapped into the OPUk payload area which is the payload area 102. The FEC area 103 is a field in which redundant data for exercising error control at data transfer time is stored.
“k” of OTUk, ODUk, and OPUk is a symbol indicative of a signal rate type and k=1, 2, 3, and so on are recommended in G. 709. An OTU3, ODU3, and OPU3 frame the OTU bit rate of which is 43.018413559 Gbit/s, an OTU2, ODU2, and OPU2 frame the OTU bit rate of which is 10.709225316 Gbit/s, an OTU1, ODU1, and OPU1 frame the OTU bit rate of which is 2.666057143 Gbit/s, and so on are standardized as OTU, ODU, and OPU frames.
For example, a plurality of SDH (Synchronous Digital Hierarchy) STM (Synchronous Transfer Mode) 16 signals, SDH STM64 signals, SDH STM256 signals, ATM (Asynchronous Transfer Mode) signals, data signals encapsulated by GFP (Generic Framing Procedure specified in the ITU-T recommendation G. 7041) or the like are multiplexed and mapped into these frames and are transferred in optical networks.
In addition, if a client signal is transferred by the use of an OTN frame, it is necessary to map the client signal into the OTN frame while absorbing the difference in signal rate (frequency) between the client signal and the OTN frame.
An asynchronous mapping method is specified in ITU-T G. 709 as a method for absorbing a difference in frequency and is widely used in communication networks.
FIG. 8 illustrates a frame format for describing the asynchronous mapping method. The size of a frame 100a is m rows×n columns. An overhead area 101a stores monitoring and control information and the like and begins from the leading column of the frame 100a. Its size is m rows×r columns. A payload area 102a into which a client signal is mapped follows the overhead area 101a and its size is m rows×(n−r) columns.
With the asynchronous mapping method a positive or negative stuff byte is inserted in bytes according to the difference in signal rate (frequency) between a client signal and a frame. In FIG. 8, a negative stuff byte area corresponding to 1 byte is ensured in the overhead area 101a and a positive stuff byte area corresponding to 1 byte is ensured in the payload area 102. 
The difference in frequency is absorbed by inserting a stuff byte or client data in the positive or negative stuff byte area. The stuff byte is fixed data such as “00000000” or “11111111.”
FIGS. 9 through 11 illustrate frequency adjustment by the asynchronous mapping method. FIG. 9 illustrates frequency adjustment made in the case of the frequency of a client signal being higher than that of a payload area of a frame. In this case, client data is stored both in a positive stuff byte area and in a negative stuff byte area in order to absorb the difference in frequency.
FIG. 10 illustrates frequency adjustment made in the case of the frequency of the client signal being equal to that of the payload area of the frame. In this case, there is no difference in frequency between the client signal and the payload area of the frame, so the client data can be mapped into the payload area of the frame. Therefore, the client data is stored in the positive stuff byte area and a stuff byte is stored in the negative stuff byte area.
FIG. 11 illustrates frequency adjustment made in the case of the frequency of the client signal being lower than that of the payload area of the frame. In this case, a stuff byte is stored both in the positive stuff byte area and in the negative stuff byte area in order to absorb the difference in frequency.
A difference in frequency which can be absorbed is found from−PJO/{m·(n−r)}<difference in frequency<NJO/{m·(n−r)}  (1)where PJO (Positive Justification Opportunity) is the number of positive stuff bytes and NJO (Negative Justification Opportunity) is the number of negative stuff bytes.
If m=4, (n−r)=3808, and PJO=NJO=1, then a difference in frequency which can be absorbed is given by−65 ppm<difference in frequency<+65 ppm  (2)
Therefore, if the difference in frequency between the client signal and the frame is in the range of −65 to +65 ppm in this example, then the client signal can be mapped into the frame.
The difference in frequency is absorbed by inserting a stuff byte in the above way. By doing so, the client signal is mapped into the frame. In this case, a stuff byte extraction process is performed at the receiving end of the frame. In order to extract the stuff byte, information indicative that the stuff byte or the client data is inserted in the positive or negative stuff byte area is stored in the overhead area of the frame and is transferred.
In the above example, 1 positive stuff byte and 1 negative stuff byte are included. Accordingly, information for indicating three stuff byte insertion states is stored in the overhead area at the sending end of the frame.
FIG. 12 indicates a stuff byte insertion state. In order to make three stuff byte insertions in bytes, information which consists of at least 2 bits should be assigned in the overhead area of the frame. For example, “11,” “00,” and “01” are assigned to Case 1, Case 2, and Case 3 respectively. By doing so, the sending end of the frame can inform the receiving end of the frame of a stuff byte insertion state.
There may be a greater difference in frequency between a client signal and a frame. In this case, the number of positive stuff bytes or negative stuff bytes is increased in order to map the client signal into the frame by the above asynchronous mapping method (usually the number of positive stuff bytes arranged in the payload area 102a is increased).
If the number of positive stuff bytes or negative stuff bytes increases, the number of information bits necessary for a stuffing process for inserting the positive stuff bytes or the negative stuff bytes also increases. The number of information bits necessary for performing a stuffing process is given bynumber of necessary bits=FLOOR(log2(number of negative stuff bytes+number of positive stuff bytes+1))+1 (bits)  (3)where FLOOR (x) is a function which omits figures after the decimal point of real number x. As can be seen from equation (3), the number of bits necessary for performing a stuffing process increases in proportion to the number of stuff bytes.
It is assumed that the number of positive stuff bytes is increased. It is assumed that when y (y<(n−r)) positive stuff bytes are arranged in the payload area 102a of FIG. 8, the y positive stuff bytes are arranged in succession in the (r+1)th column to the (r+y)th column.
At this time, at the receiving end of the frame, the frame including the y stuff bytes is received and a destuffing process (stuff extraction process) for extracting the stuff bytes is performed. However, if the y stuff bytes arranged in succession are removed, jitter or wander which causes fluctuations in the periodicity of a signal may occur (phenomenon of jitter or wander may also occur at the time of performing the stuffing process (stuff byte insertion process) at the sending end of the frame, but jitter or wander which occurs at the time of performing the destuffing process is intimately related with degradation in signal receiving characteristic).
With the asynchronous mapping method, as stated above, stuff bytes used for frequency adjustment may be inserted in succession in the payload area 102a. For example, if stuff bytes are inserted in a plurality of columns in the same row and are extracted at the receiving end of the frame, then jitter or wander occurs.
In the past, a technique for storing data corresponding to frequency deviation by which the permissible amount of absorption by negative stuff bytes is exceeded in FS bytes defined in an OTU frame was proposed as a technique for mapping a client signal into a frame (Japanese Laid-open Patent Publication No. 2008-113394).
In addition, a technique for detecting a position at which a local protocol signal is read out and assigning a stuff control amount at the time of mapping the local protocol signal into an OPU/ODU signal and a stuff control amount at the time of generating an ODTU signal on the basis of the position at which the local protocol signal is read out was proposed (Japanese Laid-open Patent Publication No. 2007-096822).
Furthermore, a technique for managing both the number of stuff bytes inserted and positions at which the stuff bytes are inserted and absorbing a difference in frequency was proposed (U.S. Pat. No. 7,020,094 B2).
In the above U.S. Pat. No. 7,020,094 (hereinafter simply referred to as the “prior art”), a technique for managing both the number of stuff bytes inserted and positions at which the stuff bytes are inserted, absorbing a difference in frequency, controlling the occurrence of jitter or wander, and mapping a client signal into a frame is disclosed.
FIG. 13 illustrates a frame format for describing the prior art. The size of a frame 50 is m rows×n columns. An overhead area 51 included in the frame 50 begins from the leading column of the frame 50 and its size is m rows×r columns. A payload area 52 follows the overhead area 51 and its size is m rows×(n−r) columns. In addition, address numbers from 1 to m×(n−r) are assigned to bytes in the payload area 52.
A client signal is mapped into the frame 50 by the prior art in the following way.
(1) On the basis of the difference in frequency between the client signal and the frame, the number of pieces of data of the client signal to be mapped into the payload area 52 is found fromCn=fclient (bits/s)×Tserver (s)/n  (4)where Cn is a number obtained by dividing the number of bits of client signal data to be mapped into the payload area 52 by n, fclient is the bit rate (bits/s) of the client signal, and Tserver is the signal period (s) of the frame. If n=8, then Cn represents the number of bytes of the client signal data to be mapped into the payload area 52. For the sake of simplicity it is assumed that n=8 in the following description.
(2) Whether the client signal data or a stuff byte is inserted into each byte in the frame is determined on the basis of:N×Cn mod(m×(n−r))<Cn  (5a)N×Cn mod(m×(n−r))≧Cn  (5b)where N is an address number assigned to each byte in the payload area 52 and (m×(n−r)) is the total number of bytes in the payload area 52. A left side in each of inequalities (5a) and (5b) means a remainder obtained by dividing (N×Cn) by (m×(n−r)), where Cn is assumed as C8.
In the case of inequality (5a), the remainder is smaller than Cn. In this case, client data is inserted at address N. In the case of inequality (5b), the remainder is greater than or equal to Cn. In this case, a stuff byte is inserted into at address N.
FIG. 14 illustrates an example of how stuff bytes and client data are arranged. With the prior art a stuff byte or client data is inserted into the payload area 52 of the frame on the basis of the value of Cn. In this case, jitter or wander may occur at the time of extracting the stuff byte at the receiving end of the frame. If stuff bytes are arranged in succession, jitter or wander may occur. Accordingly, stuff bytes are inserted into the payload area 52 so that they will be distributed uniformly in the payload area 52.
In order to perform a destuffing process at the receiving end of the frame, Cn indicative of the number of bytes of the client signal inserted into the payload area 52 is stored in the overhead area 51 of the frame and is transferred at the sending end of the frame.
At this time the value of Cn is in the range of 0 to (m×(n−r)). Therefore, in order to represent this value, bits the number of which is calculated byFLOOR(log2(m·(n−r))+1 (bits)are used.
With the above prior art, as has been described, the positions at which stuff bytes are inserted into the payload area 52 are calculated by the use of inequalities (5a) and (5b). By doing so, the stuff bytes are inserted into the payload area 52 so that they will not be arranged in succession. As a result, the occurrence of jitter or wander is controlled at the time of extracting the stuff bytes at the receiving end of the frame.
With the prior art, however, insertion determination (determination of whether to insert client data or a stuff byte) is made for each byte of the payload area 52 by the use of inequalities (5a) and (5b). As a result, the number of operations performed for the insertion determination is large and frame transmission performance deteriorates. If the frequency of the frame becomes higher, then the number of operations performed for the insertion determination increases greatly. Accordingly, it may be difficult to realize the insertion determination.
The problem with the prior art will now be described by the use of concrete numeric values. FIG. 15 illustrates an example of operation performed by the use of the prior art. It is assumed that the size of a payload area 52a of a frame is 24 bytes (in FIG. 15, a number indicated in a square frame is an address N assigned to each byte). In addition, it is assumed that the number Cn of bytes of client signal data to be mapped into the payload area 52a is 17.
In order to map client data corresponding to Cn=17 bytes and stuff bytes into the payload area 52a the size of which is 24 bytes, insertion determination is made by the use of:N×17 mod 24<17  (5a-1)N×17 mod 24≧17  (5b-1)
If inequality (5a-1) is satisfied, then client data is inserted at address N. If inequality (5b-1) is satisfied, then a stuff byte is inserted at address N.
To be concrete, when N=1, 17 mod 24=17 (≧17). Accordingly, a stuff byte is inserted at address 1. When N=2, 34 mod 24=10 (<17). Accordingly, client data is inserted at address 2.
When N=3, 51 mod 24=3 (<17). Accordingly, client data is inserted at address 3. When N=4, 68 mod 24=20 (≧17). Accordingly, a stuff byte is inserted at address 4. When N=5, 85 mod 24=13 (<17). Accordingly, client data is inserted at address 5.
Similarly, when N=23, 391 mod 24=7 (<17). Accordingly, client data is inserted at address 23. When N=24, 408 mod 24=0 (<17). Accordingly, client data is inserted at address 24.
As indicated in a payload area 52a-1, the client data and the stuff bytes are inserted as a result of the above insertion determination. The stuff bytes are arranged uniformly, so the occurrence of jitter or wander can be controlled, for example, at the time of extracting the stuff bytes.
However, the insertion determination is made for each byte in the payload area, so a very large number of operations are performed. In this example, the size of the payload area 52a is 24 bytes. Accordingly, in order to make the insertion determination for each byte in the payload area 52a, it is necessary to perform a total of 24 operations. With the prior art a large number of operations are performed in this way for making the insertion determination. As a result, a signal cannot be mapped efficiently and frame transmission performance deteriorates.